Multiple sclerosis is one of the most common chronic neurologic diseases in the United States today. More than 100,000 patients are severely crippled by this disorder. The cause of the disease is not known and there is no effective long-term treatment. Since Charcot originally described multiple sclerosis more than 100 years ago, neurologists and neuropathologists have noted an expanded spectrum of clinical and pathologic variation from individual to individual afflicted with multiple sclerosis. In the 1950's, epidemiologic studies indicated both high and low-risk prevalence areas for multiple sclerosis. Further studies on migrant populations suggested that multiple sclerosis was acquired in childhood although clinical disease is not manifested until adult life. While these data were accumulating, the concept of the "slow-virus" neurologic disease was developing, with apparent incubation periods of many months or years. The concept that multiple sclerosis might possibly be a viral disease with a long incubation period was an attractive hypothesis born by the blending of fruitful multiple sclerosis epidemiologic studies with the knowledge that the spongiform encephalopathies (including human Jakob-Creutzfeldt) were transmissable and also that other chronic neurologic diseases of man such as subacute sclerosing panencephalitis (SSPE) and progressive focal leukocephalopathy (PML) were directly associated with virus infections.